This invention relates to lubricating oil compositions and more particularly to those which have excellent anti-wear properties and a long fatigue life, suitable for transmissions for automobiles.
An automatic transmission for automobiles comprises a torque converter, a planetary gear unit, bearings, a wet clutch, and a hydraulic control unit controlling these components. However, in recent years, automatic transmissions have been susceptible to more sever load than ever due to the progresses of the development of high-powered engines and of the downsizing of automatic transmissions. Lubricating oils to be filled into such transmissions, i.e., transmission oils are required to have excellent extreme pressure properties and anti-wear properties while maintaining a high lubricity and a long fatigue life which is an ability to prevent pitching or flaking (defects at the lubricated surface because of being damaged) in bearings and gears for a long life.
In order to meet such requirements, it is known that for example lubricating oils such as automotive transmission oils are blended with sulfur- or phosphorus-based additives having excellent extreme pressure properties and anti-wear properties. While sulfur-based additives are excellent in extreme pressure properties, they can not avoid wears caused by corrosion and abrasion due to their strong activity to metal surfaces, leading to a problem when they are used alone. On the other hand, phosphorus-based additives are less in wear caused by corrosion and abrasion due to their weaker activity to metal surfaces, than the sulfur-based additives but often have problems due to the lack of extreme pressure properties to avoid pitching or flaking when they are used alone in automatic transmissions where extreme pressure properties are required to be exhibited under severe conditions.